1. Field of the Invention
The present invention relates to an aluminum alloy sheet for a cross fin and a process of producing same.
2. Description of the Related Art
Heat exchangers are essentially composed of a heat media pipe for circulation of heat transfer media, such as water or air, and a fin placed in close contact with the pipe to exchange heat with the environment, and are manufactured by various types of production processes.
Heat exchangers produced by bonding a heat media pipe and a fin by soldering have a strong bond resistance to vibration and are suitably used for the radiator of automobile but have a drawback that the bonding process requires a high temperature close to 600.degree. C. to melt a solder and the production cost is therefore increased.
Heat exchangers designed for air conditioners or refrigerators are produced by bonding a heat media pipe and a fin by a mechanical process, which is less expensive than soldering. The fin is made from a thin sheet having a thickness of 100 to 150 .mu.m by forming therein a through hole having a diameter corresponding to that of a heat media pipe, and the pipe is then fitted in the through hole to bond the pipe and the fin. This type of fin having a through hole in which a heat media pipe is to be fitted is called "cross fin". A large number of cross fins are stacked at a space therebetween, for example, 1 to 2 mm, with the through holes thereof being aligned in an array for engagement with the pipe to form a heat exchanger.
The through hole of the cross fin is formed so as to have a circumuferential wall or a collar which extends from, and perpendicularly to, the fin surface to a height and has an edge lip or a flare radially extending outward in order to ensure provision of a selected space between the fins when stacked and a good mechanical bond and heat transfer between the fin and the pipe.
Because of a good formability and heat conductivity, aluminum and aluminum alloys are used as a material of cross fins. The collar having an outward extending edge flare is typically formed by the following processes I, II, or III.
I. The draw process, in which a sheet of aluminum or an aluminum alloy (hereinafter simply referred to as "aluminum sheet") is first deep-drawn to form a bulge having a diameter greater than that of the heat media pipe and a large height, further deep-drawn in three or four steps to reduce the diameter and the height of the bulge to define a collar by an annular wall of the bulge, pierced (to open a through hole) and burred (to expand the through hole) to form a through hole for engagement with the heat media pipe, and finally reflared (to extend the collar end radially outward).
II. The drawless process, in which an aluminum sheet is first pierced and burred to form a through hole, then ironed in two steps to reduce the sheet thickness and form a collar portion, and finally reflared.
III. The combination process, in which a sheet is first deep-drawn by the first one or two steps of the draw process, pierced and burred to form a through hole having a collar lower than a designed height, ironed to provide the collar with a correct height, and finally reflared.
The draw process if mostly carried out by deep drawing, requiring good ductility to the work, and uses a soft aluminum sheet.
The drawless process forms a collar by ironing and allows the use of a hard aluminum sheet. However, the ironing die is subjected to significant wear under the presence of a volatile lubricant.
The combination process reduces the die wear by combining deep drawing and ironing.
The cross fins are conventionally produced by these processes from an aluminum sheet, which has a good heat conductivity and formability.
To provide an aluminum sheet having an improved strength and bore expandability, Japanese Unexamined Patent Publication (Kokai) No. 5-156412 proposed a process in which a continuous-cast sheet containing 0.01 to 0.15 wt % Si, 0.05 to 0.040 wt % Fe, 0.10 to 0.50 wt % Mn, and the balance consisting of Al and unavoidable impurities is cold-rolled at a reduction of 80% or more and the cold-rolled sheet is temper-annealed at a temperature of 230 to 330.degree. C.
To provide an aluminum alloy sheet having good strength, ductility and formability, Japanese Unexamined Patent Publication (Kokai) No. 8-327291 proposed a process in which a continuous-cast and rolled sheet containing Fe and Ti in selected amounts is intensely cold-rolled and temper-annealed to bring a selected amount of Fe into solid solution and also to provide a combined microstructure composed of subgrains and recrystallized grains mixed with subgrains in the mid-thickness zone of the sheet.
The fin forming process including the aforementioned deep-drawing, reflaring, and ironing is a precise die forming process in which a low friction lubricant is necessary to ensure good formability. The low friction lubricant has a viscosity as high as 7 to 8 cSt and requires a special treatment agent for the removal thereof from the formed fin, causing an unacceptable pollution of the environment.
To avoid the use of a special treatment agent for removal of lubricant, it has been proposed to use a volatile lubricant such as gasoline, which, however, causes the forming to be difficult.
The aluminum sheet produced by the process of Japanese Unexamined Patent Publication (Kokai) No. 5-156412 has a poor bore expandability and application flexibility in view of the recent antipollution requirements. In the heat exchangers designed for air conditioners or refrigerators, the heat media pipes have various sizes and the aluminum sheet for the fin use must has a high formability sufficient to provide a large fin pitch (or a reflared collar having a large height) fittable to any size of heat media pipes. The aluminum sheet of Japanese Unexamined Patent Publication (Kokai) No. 5-156412, when used with a volatile lubricant, has a poor fin formability, causes cracking to occur at the collar end during reflaring, cannot be used when a heat media pipe has a small outer diameter because the bore expandability with respect to the outer diameter is necessarily large, fails to provide good bond to the heat media pipe, and provides a poor appearance.
The aluminum sheet produced by the process of Japanese Unexamined Patent Publication (Kokai) No. 8-327291 has a poor temper annealing property, i.e., has a large scatter of strength when temper-annealed under a wide selection of the temper annealing conditions including heating temperature and time.